I received this reply from Luxembourg last year but never made a blog post about it. I had written to Grand Duke Henri & Grand Duchess Maria Teresa.
The front of the envelope is different than all my past replies from the Luxembourg royals. Instead of the usual stamp and postmark, this is franked. Many monarchies have switched to this over the years as it makes it much easier and usually cheaper. Envelopes can be "franked" by a machine, stamped with this box with their account number. And then Post Luxembourg just charges them for the postage, which is usually given at a reduced rate.
The card says:
Luxembourg, December 2021
Their Royal Highnesses the Grand Duke and The Grand Duchess
of Luxembourg, very touched by your kind greetings, send you their most
heartfelt wishes for Christmas and the New Year.